Ink jet printers, that is to say, printers which form an image by firing a plurality of discrete drops of ink from one or more nozzles on to the surface of a recording sheet placed adjacent the nozzles, have recently enjoyed a large increase in sales. Such ink jet printers have the advantage that they can reproduce good quality text and images, in both monochrome and full color, can produce both reflection prints and transparencies, and are relatively inexpensive to manufacture and to operate. Accordingly, ink jet printers now dominate the home/small office market, and are often also used to provide color capability not available from the monochrome laser printers typically employed in larger offices.
Although modern ink jet printers can print on almost any conventional paper or similar medium, and indeed are routinely used with commercial photocopying paper for printing text, the quality of images produced by such printers is greatly affected by the properties of the medium used. To produce high quality images reliably, it is necessary that the medium (ink jet recording sheet) dry rapidly since otherwise the ink is likely to smear when successive sheets are stacked in the output tray of the printer. On the other hand, the medium should not promote excessive spreading of the ink droplet, since such spreading reduces image resolution and may result in color distortion if adjacent ink droplets intermix. The medium also should not promote “wicking”, that is to say, spreading of ink by capillary action through fibrous media, such as paper. The medium must be capable of absorbing the ink without substantial distortion of the medium, since otherwise unsightly “cockling” (formation of ripples and similar folds) may occur, and most observers find such distortions unacceptable. Once the ink has dried, the medium should be such that contact of the image with moist surfaces (such as sweaty fingers) does not result in bleeding of ink from the image. Finally, since the surface characteristics, such as smoothness, glossiness and feel, of the image are largely determined by the same characteristics of the medium, the medium should possess characteristics appropriate to the type of image being printed. When, as is increasingly common, an ink jet printer is used to print a digital image produced by a camera or a scanner, the medium should be smooth and possess the high gloss and smooth feel of conventional silver-halide based photographic printing paper.
There are two types of ink jet medium, i.e. reflection type displays (prints) and transmission type displays (transparency). Substrate used for prints in general are coated paper or resin coated paper. Substrate used for transparency in general are plastic films, such as cellulose acetate and polyesters. To improve the affinity of the ink with the medium and to improve the image quality and the durability of the prints, water soluble polymers with or without pigments are commonly used. Polyvinyl alcohol and polyvinyl pyrrolidone are among the most common polymers used for the inkjet recording materials.
Copolymers with vinylpyrrolidone are known. Poly(vinylpyrrolidone-co-vinyl acetate), a product of copolymerization of vinylpyrrolidone and vinyl acetate was the first commercially successful class of copolymer of vinylpyrrolidone and is currently manufactured in commercial quantities by both ISP Chemical Corporation (ISP) and BASF AG (BASF). Copolymers of vinylpyrrolidone with various other monomers are also known. The best known include dimethylaminoethyl methacrylate (DMAEMA), methylvinylimidazolium chloride (Polyquaternium 16), methacrylamidopropyltrimethyl ammonium chloride (Polyquaternium 28), acrylic acid (AA), alpha-olefins, and styrene. (Kirk-Othmer Encyclopedia of Chemical Technology, N-Vinylamide Polymers: 7. Copolymerization, http://www.mrw.interscience.wiley.com/kirk/articles/vinylogi.a02/sect17.html.) However, these copolymers do not have adequate image quality and usually have poor smudge and finger print resistance.